This invention relates to a phase shift photomask for use in the microfabrication of high density integrated circuits such as LSI and VLSI and magnetic heads.
Photolithography using photomasks is utilized in the microfabrication of semiconductor integrated circuit devices such as LSI and VLSI. The photomasks used are generally of the following construction. A light shielding film, typically a chromium film is formed on a transparent substrate such as quartz glass, and a chromium oxide film is formed on the chromium film for preventing light reflection, which gives a photomask blank. A predetermined pattern is defined in the light shielding film of this photomask blank.
As the degree of integration of semiconductor integrated circuit devices increases, the micropatterning size is reduced. This gives rise to several problems including narrowing of the light shielding zone due to the light diffraction effect upon exposure, distortion of the light shielding zone pattern due to the interference effect between adjacent patterns, and difficult processing of microscopic configuration. To overcome these problems, a photomask having a phase shifter utilizing the coherency of incident light was proposed (see JP-A 58-173744, JP-B 62-59296 and JP-A 7-110572). The phase shifter includes several types. With respect to transmissive masks, there are known the halftone type in which a phase difference of xcfx80 is provided between the light transmitted by the shielding film and the light transmitted outside the shielding film so that the attenuation of light at the edge of the shielding film is enhanced (see JP-A 4-136854) and the Levenson type in which a phase shifter is formed on one of a pair of adjacent light transmissive zones (see JP-B 62-59296).
One exemplary reflective mask is shown in Japanese Patent No. 2,889,047 as comprising at least two reflective layers wherein a phase difference due to reflection from different heights is utilized.
These phase shift masks, however, encounter difficulty in manufacture process because precise uniformity is imposed on their film thickness and quality and a phase difference must be strictly controlled. For example, a reactive sputtering process is generally used in the manufacture of a phase shifter film. On an attempt to deposit by reactive sputtering Moxe2x80x94Sixe2x80x94Oxe2x80x94N which is disclosed as a phase shifter in JP-A 7-140635, there arises a problem that the thickness of an optical film becomes uneven in plane, failing to meet the uniformity required for the phase shifter.
Additionally, the optical constant of the film is restricted. Even in the case of a high refractive index film which can be used at a reduced thickness, its thickness is determined from the refractive index, and its coefficient of absorption is determined from the transmittance. The film which can be used as a phase shifter is restricted. With respect to the photomask using a phase shifter, it is known from JP-A 6-308713 that when the phase shifter consists of a single layer film, the film thickness d must satisfy the equation:
(nxe2x88x921)d=xcex/2
wherein xcex is the wavelength of incident light and n is the refractive index of the film. This indicates that light is transmitted by the phase shifter without undergoing multiple reflection.
For this reason, once a refractive index is determined, the thickness of a phase shifter film is determined. It is impossible to reduce the thickness of a phase shifter film. This results in low productivity, mask deflection, and film peeling.
An object of the invention is to provide a phase shift photomask which can reduce the thickness of a phase shifter film and minimize the variation of phase difference.
It has been found that if light undergoes multiple reflection within a phase shifter film, the phase variation relative to a variation of film thickness can be reduced, and hence, optical restrictions on the film can be alleviated.
The invention provides a phase shift photomask comprising a substrate transmissive to exposure light and a phase shifter formed thereon. The region of the substrate which is not covered by the phase shifter defines a first light transmissive region, and the phase shifter defines a second light transmissive region. According to the invention, exposure light undergoes multiple reflection within the phase shifter. Preferably the phase shifter includes at least two stacked films, and more preferably at least a reflective film and a transparent film.
The phase shifter in the photomask of the invention is to place a stationary point at a phase difference of 180 degrees utilizing multiple reflection of light. The structure that multiple reflection occurs within the phase shifter has the following benefits.
(1) At a film thickness providing a phase difference a, the dependency of phase difference on (optical) film thickness is reduced, and even the phase difference is substantially independent of the film thickness. Then the variation of phase difference is minimized even with variations of film thickness and quality, so that the film can be easily formed.
(2) Any film which is transmissive to incident light can be used. Even a film having a too low coefficient of absorption is useful. A film can be chosen in terms of film stress other than optical properties.
(3) Because of multiple reflection of light within the film, the film thickness can be reduced.